Most anyone who has traveled outside of the United States, or is from outside the United States, has likely had the opportunity to note the almost painfully monolinguistic nature of the average American.
If it's not English, we Americans don't want to hear it. If we must go abroad, the presence of other languages is simply an inconvenience we are forced to deal with. With a little persistence, we will badger the locals into learning English on the spot for our benefit if it comes to that.
In all seriousness, it is true that most Americans are ill equipped to function outside the English speaking world. For good or ill, this is the reality and is understandably of concern to many Americans heading out of the country. For those who wish to venture outside our anglicized borders, but are intimidated by communications barriers, Puerto Rico is a great destination.
It has been said that Puerto Rico is one of the most solidly bilingual places in the world. It's hard to say whether this is true or not, but milling about San Juan certainly lends credence to that argument. By far, most of the people I met or came in contact with in San Juan were fluent in English. As often as not, these folks spoke perfect English with a typical American accent.
Of course, if you speak Spanish there is no reason not to. Even if you are not fluent, it would certainly seem respectful to at least attempt some rudimentary usage of their language. In any case, however, you will find the sanjuaneros are generally more than willing to lend their friendly assistance in English if that's how you will best understand them.
Thus, San Juan gives you the best of both worlds, in a sense. While you have all the benefits of visiting a place with a long, rich Spanish cultural heritage, there is also a linguistic safety net for we language impaired gringos. This should certainly make Puerto Rico a viable and alluring option for even the most timid of travelers.